To reproduce a color original, an electronic color scanner is used to separate the image into its cyan, magenta and yellow components using red, green and blue filters. A combination of these is then used to create the black component. The output of the scanner consists of a halftone screen for each of the four colors of the original with graduated dot sizes reproducing the tonal range of the original. Printing plates are then made from these screens which are then used to print the image on paper using the four process inks.
Predictable and consistent color is of paramount importance to printers. To ensure color control and to maintain a consistent printed product, the ink film thickness and the size and color strength of these halftone dots must be monitored and controlled.
The inks used are for the most part transparent, and act as color filters. When white light is shining on a green ink on white paper it is seen as green since the blue and red parts of the spectrum are absorbed as the light passes through the ink. The transmitted light is reflected by the white paper and is filtered a second time as it reemerges from the ink surface (see FIG. 1). The thicker the ink film, the more light the ink absorbs and the darker the ink seems. Thus, a thick ink appears to have a greater density.
In the halftone process the tonal scale of the image is represented with dots of differing sizes. The assumption is that the ink film thickness of each dot is the same irrespective of its size or diameter. Since it is difficult to separate the individual components that create the image, in order to measure the ink film thickness and to ensure its uniformity across the press sheet, a series of control test strips of solid ink, i.e. 100% dot, for each color are placed in the color bar and are measured with the reflection densitometer.
These test strips as shown in FIG. 2, called print control strips or color bars, are available commercially from various vendors, and consist of strips of film containing the various test elements for each of the four colors. In some cases six color versions are available when special colors might be used. The usual densitometric targets in a colorbar are: Solid Ink Density, Dot Area/Gain half and three-quarter tints, Contrast and the Trapping of ink overprints.
The densitometer emits white light (approximately equal amount of red, green and blue) which is focused onto the printed surface and collects light reflected from the print. The densitometer subtracts the amount of light returned from the amount of light shone to determine how much was absorbed. The reflected light is received by a photo diode and then converted into an electrical signal. The electrical current is compared to a reference value (from white light) and the difference is used to calculate the film reflectance and hence the optical density.
In the United States, it has been the custom to use Status T (wide band) filters, while in Europe Status E filters are used. Therefore in order to compare O.D. results, the same type of calibrated filter must be used. The filters used to read process colors are red for cyan ink, green for magenta ink, and blue for yellow ink. Typical status T O.D. values for solid ink density are 0.90 to 1.10 for yellow, 1.10 to 1.40 for magenta, 1.25-1.45 for cyan and 1.55 to 1.85 for black.
The densitometer within certain limitations, gives higher density readings with increasing ink film thickness. When the ink film thickness approaches a certain point, however, there is no further increase in density and any extra ink added is wasted.
Ink usage efficiency, most often referred to as ink mileage, and its press performance are very important criteria for assessing the value of inks. Press mileage is usually expressed as lbs or Kg of ink required to print 1000 prints at a given SWOP Optical Density (O.D). In order to obtain accurate ink mileage on the press the printers have to measure the amount of ink used to print at least 500,000 prints, preferably 1,000,000 prints. This method is time consuming, has poor precision due to lack of repeatability, which would be very costly to determine, and press trials are expensive. Press mileage could vary significantly due to variation in color saturation of the prints which can differ by as much as 10% within a given SWOP range. Improper start up procedures, changes in ink/water balance, or changes in ink tack and body can cause excessive waste of paper and print density variation and thus decrease the accuracy of press mileage.
It is important to note that the densitometer readings are on a logarithmic scale. A density of zero indicates that 100% of the incident light is being reflected. At a density of 1.00 only 10% of the light is being reflected, and at a density of 2.00 only 1% of the light is being reflected. The O.D may be defined as follows:
      O    .    D    .    =      log    ⁡          (              100                  %          ⁢                                          ⁢          R                    )      where R is the reflectance.
High print densities can lead to poorer ink mileage due to non linearity between the weight of ink film and light absorption by the colorant. Densitometers also have difficulty in measuring accurately very light spot colors (low O.D.).
The arrangement of the components of a typical reflection densitometer is illustrated in FIG. 3.
Hamada et al. (Pulp and Paper Research Conference, Tokkyo, Japan, 18-19 Jun. 2001, pp 152-155, Japan TAPPI) disloses that the amount of printed cyan ink can be determined by measuring copper intensity using X-ray fluorescence. Specifically, Hamada et al. discloses that the content of copper present in a cyan ink printed on paper can be mesured as an intensity of the peak in the X-ray fluorescence spectrum. However, it is difficult to use this method to quantitate small amounts of printed ink (less than 1 g of ink/1 m2 of printed paper). In addition, calculating printing ink efficiency using this method is not practical or cost efficient and is only limited to inks with copper additives such as cyan ink which contains copper-phthalocyanine as the blue pigment.
In view of the foregoing, it is therefore desirable to provide analytical methods for measuring ink usage efficiency (ink mileage) in commercial printing processes accurately, faster and at lower cost, based on spectrophotometric principles whereby the total chromophore content of the taggant per unit area of print as well as the colorant content of the ink used in the print is measured. The method can be applied to all types of printing inks including heatset, UV flexo and publication gravure. The ink mileage test method of the present invention can be used as quality control tool to accurately predict press mileage, for the development of improved inks and colorants at a faster rate to increase the speed of new products to market and to predict more accurately the cost of printing. The cost to print is defined as dollars needed to print 1000 square meters of print at mid SWOP O.D.Cost of print=$/1000 m2 